1. Field of Invention
The present invention relates to a motor control device that detects an overload imposed on a motor.
2. Description of Related Art
When a main axis that drives a cutting tool of a machine tool is successively subjected to an excessive load, the main axis may be damaged. Therefore, it is common to monitor the load torque imposed on a motor that rotates the main axis. When the load torque exceeds a reference value, the motor is stopped so as to prevent the main axis from being damaged.
Japanese Laid-open Patent Publication No. 5-116094 discloses a technique to obtain an estimated disturbance torque from a disturbance estimation observer, and to output an alarm when the estimated disturbance torque exceeds a reference value. In addition, Japanese Laid-open Patent Publication No. 2003-80529 discloses a technique to output an alarm and stop the motor, when the detected load torque exceeds a predetermined upper limit or lower limit.
In this regard, when the cutting tool starts to cut a workpiece, the load torque imposed on the motor driving the main axis momentaneously increases. In such a case, the load torque imposed when the cutting is started exceeds the reference value, and therefore the motor should be stopped.
In other words, when the reference value is set to a lower value, the load torque may exceed the reference value since the load torque increases at the time that the cutting is started. Accordingly, it is desirable to disregard the excessive load that momentaneously arises in such a case, to thereby continue the operation of the machine tool.
The present invention has been accomplished in view of the aforementioned situation, and provides a motor control device configured to stop the motor only when the motor is successively subjected to an excessive load.